A generalised anti-skid system is shown in FIG. 1 of the accompanying drawings. A sensor 10 is used to detect acceleration or deceleration of the wheel 20. The sensed information is passed to an electronic control unit 30 which in turn regulates the control valve 40 which controls the required wheel braking. If the wheel deceleration exceeds that which is physically possible for the vehicle, then the ECU 30 signals the control valve 40 to reduce fluid pressure to the brakes on the wheel 20. As the wheel 20 accelerates back towards the vehicle's speed the braking pressure is increased again. The cycle is repeated for the period during which input fluid pressure to the brakes would cause the wheel 20 to lock without the anti skid system. One or more wheels of the vehicle may be sensed and the brakes of one or more wheels may be controlled. Furthermore the number of controlled wheels need not equal the number of sensed wheels. The control valve 40 may have a number of different states, e.g. pressure reduction, pressure hold, slow pressure increase, rapid pressure increase (normal braking), etc.
The design of an ECU is dictated by a number of requirements. One requirement is that the control strategy of an ECU must give very good performance under almost all possible road conditions. As a rough guide, the stopping distance on a vehicle having an anti-skid system should be no worse than that of a locked wheel stop. Legal requirements apply in some countries with regard to this requirement. Secondly, an ECU must be failsafe to a very high degree, i.e. in the perfect example a system failure of any kind must, at worst, restore normal braking to the vehicle. A third requirement is that if there occurs external to an ECU an electrical failure which affects the system, the driver of the vehicle to which the anti-skid system is fitted must be alerted by some means. Usually a warning light is fitted for this purpose. Examples of external failures are disconnected sensors or disconnected solenoids which form part of the control valve. Fourthly, the driver of the vehicle must be informed if a failure occurs within the ECU itself. A further requirement is that the ECU must be highly resistant to electromagnetic interference. Again legal requirements apply in some countries with regard to this requirement.
Early ECU's for anti-skid systems were based on analogue electronic techniques. Arrangements of frequency to voltage converters, ramp generators, comparators, differentiators, etc. were used to establish brake control decisions. More recent designs have been based on custom digital integrated circuits, while the latest designs tend to be microcomputer based.
Microcomputer based electronic control units can have as complex a control cycle as is required for optimum performance. Present and anticipated legislative requirements may be met with such an ECU. Temperature variations do not present a problem, as suitable microcomputers may be used which can operate within the required temperature range of -40.degree. C. to 85.degree. C. Significant adaptability can now be achieved limited only by the read/write (RAM) memory size and software designer ingenuity. Thus, good performance can be achieved for practically all possible road conditions. A significant advantage in microcomputer based systems is that variations in the control strategy are easily implemented in the product development phase. This results in reduced product development time and/or a more optimised control strategy in the final product.
Using a microcomputer based ECU, it is possible to obtain a very good failsafe capability. The microcomputer may itself be duplicated and comparisons made between the two sets of control decisions. If a discrepency arises, the system is shut down by independent hardware. Such a system is very reliable but is somewhat expensive.
With a microcomputer based ECU, there is no difficulty in meeting the external electrical failure requirement. Due to the multifunctional nature of a microcomputer, the main operating hardware may be largely utilised for this function.
Total duplication in a microcomputer based ECU can give fault determination capability.
Microcomputer based ECU's, because of logic level noise immunity, tend to be totally resistant to moderate levels of electromagnetic interference. Under extreme conditions, however, a bit error may lead to complete loss of direction in program execution. When this occurs the system usually does not resume normal operation even when interference is removed.